Lubricating oil composition, shock absorber, and method for using lubricating oil composition

ABSTRACT

A lubricating oil composition may be used for lubrication of a shock absorber, and may include a base oil (A), a polyalkyl (meth)acrylate (B) having a weight average molecular weight in a range of from 150,000 to 900,000, and an olefin copolymer (C) having a weight average molecular weight of 100,000 or less.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition, a shockabsorber using the lubricating oil composition, and a method for usingthe lubricating oil composition.

BACKGROUND ART

A shock absorber is used by being filled with a lubricating oilcomposition for a shock absorber, and is a mechanism mounted on a carbody in order to generate a damping force that damps the vibration ofthe car body, to optimize the friction characteristics of the slidingportion to control the riding quality of the car body, to suppress thefriction wear of the sliding portion to secure durability, and the like.

Various lubricating oil compositions for a shock absorber that may besuitably used in such a shock absorber have been developed.

For example, Patent Literature 1 discloses an invention relating to alubricating oil composition for a shock absorber that contains alubricating base oil having a predetermined kinematic viscosity, anon-dispersed poly(meth)acrylate-based viscosity modifier having aweight average molecular weight of 30,000 to 200,000, and a primary zincdialkyldithiophosphate and a secondary zinc dialkyldithiophosphate in apredetermined content ratio.

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 2018-203953

SUMMARY OF INVENTION Technical Problem

According to the studies by the present inventors, the lubricating oilcomposition described in Patent Literature 1 has been found to beinsufficient in an effect of suppressing cavitation. In addition, alubricating oil composition used for lubrication of a shock absorber inwhich not only the effect of suppressing cavitation, but alsocharacteristics such as cold startability and shear stability areimproved is required.

Solution to Problem

The present invention provides a lubricating oil composition used forlubrication of a shock absorber containing a base oil, a polyalkyl(meth)acrylate having a predetermined weight average molecular weight,and an olefin copolymer having a predetermined weight average molecularweight. Specifically, the present invention provides the followingembodiments [1] to [10].

-   -   [1] A lubricating oil composition used for lubrication of a        shock absorber, comprising: a base oil (A), a polyalkyl        (meth)acrylate (B) having a weight average molecular weight of        150,000 to 900,000, and an olefin copolymer (C) having a weight        average molecular weight of 100,000 or less.    -   [2] The lubricating oil composition according to the above [1],        wherein the total content of the component (B) and the        component (C) is 20.0 mass % or less based on the total amount        of the lubricating oil composition.    -   [3] The lubricating oil composition according to the above [1]        or [2], wherein a content ratio [(B)/(C)] by mass of the        component (B) to the component (C) is 1/99 to 90/10.    -   [4] The lubricating oil composition according to any one of the        above [1] to [3], wherein a content of the component (B) is 0.1        to 10.0 mass % based on the total amount of the lubricating oil        composition.    -   [5] The lubricating oil composition according to any one of the        above [1] to [4], wherein a content of the component (C) is 0.1        to 10.0 mass % based on the total amount of the lubricating oil        composition.    -   [6] The lubricating oil composition according to any one of the        above [1] to [5], wherein a weight average molecular weight of        the component (C) is 8,000 or more and 100,000 or less.    -   [7] The lubricating oil composition according to any one of the        above [1] to [6], wherein the component (C) comprises an        ethylene propylene copolymer (C1).    -   [8] The lubricating oil composition according to any one of the        above [1] to [7], wherein the total content of the        components (A) to (C) is 80 to 100 mass % based on the total        amount of the lubricating oil composition.    -   [9] The lubricating oil composition according to any one of the        above [1] to [8], wherein a BF viscosity of the lubricating oil        composition at −40° C. is 1,500 m·Pas or less.    -   [10] Use of a lubricating oil composition, comprising applying        the lubricating oil composition according to any one of the        above [1] to [9] to lubrication of a shock absorber.

Advantageous Effects of Invention

The lubricating oil composition of one preferred embodiment of thepresent invention is excellent in characteristics such as coldstartability, shear stability, and an effect of suppressing cavitation,and the lubricating oil composition of the particularly preferredembodiment is excellent in all of the cold startability, the shearstability, and the effect of suppressing cavitation, and therefore, itcan be suitably applied to lubrication of a shock absorber.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a schematic diagram of a testing apparatus for evaluating theeffect of suppressing cavitation.

DESCRIPTION OF EMBODIMENTS

In the present specification, a kinematic viscosity and a viscosityindex mean values measured and calculated in accordance with JISK2283:2000.

In the present specification, the weight average molecular weight (Mw)and the number average molecular weight (Mn) are values in terms ofstandard polystyrene measured by gel permeation chromatography (GPC),and specifically mean values measured by the method described inExamples.

In the numerical range described in the present specification, upperlimit and lower limit values can be arbitrarily combined. For example,when a description of “preferably 30 to 100, more preferably 40 to 80”is described as a numerical range, ranges such as “30 to 80” and “40 to100” are included in the numerical range described in the presentspecification. For example, when a description of “preferably 30 ormore, more preferably 40 or more, and preferably 100 or less, morepreferably 80 or less” is described as a numerical range, ranges such as“30 to 80” and “40 to 100” are also included in the numerical rangedescribed in the present specification.

In addition, for example, a description of “60 to 100” as the numericalrange described in the present specification means a range of “60 ormore and 100 or less”.

[Constitution of Lubricating Oil Composition]

The lubricating oil composition of the present invention contains a baseoil (A), a polyalkyl (meth)acrylate (B) having a weight averagemolecular weight of 150,000 to 900,000, and an olefin copolymer (C)having a weight average molecular weight of 100,000 or less.

The lubricating oil composition used for a shock absorber requiresvarious characteristics. As one of such characteristics, the effect ofsuppressing cavitation is required. Cavitation refers to a physicalphenomenon in which generation and extinction of bubbles occur in ashort time due to the pressure difference in the flow of the lubricatingoil composition, leading to a factor for causing deterioration ofresponsiveness of shock absorbers and noises. For example, thegeneration of cavitation in a shock absorber mounted on a vehicleaffects the ride quality of the vehicle and the like.

In addition, the lubricating oil composition used for a shock absorberrequires not only the effect of suppressing cavitation, but alsocharacteristics such as cold startability and shear stability.

The present inventors have intensively studied to obtain a lubricatingoil composition in which these characteristics are improved in goodbalance, and have found that a combination of a polyalkyl (meth)acrylate(B) and an olefin copolymer (C) each having a predetermined weightaverage molecular weight is useful.

That is, it has been found by the studies of the present inventors thatthe component (B) is a factor that may improve the effect of suppressingcavitation and cold startability, but causes a reduction of the shearstability, whereas the component (C) is a factor that may improve theeffect of suppressing cavitation and shear stability, but causes areduction of the cold startability.

Thus, the present inventors have found that a lubricating oilcomposition in which cold startability, shear stability, and the effectof suppressing cavitation are improved in good balance can be obtainedby using the components (B) and (C) in combination. The presentinvention has been made based on the findings.

In the lubricating oil composition of one embodiment of the presentinvention, from the viewpoint of obtaining a lubricating oil compositionin which cold startability, shear stability, and the effect ofsuppressing cavitation are improved in good balance, the content ratio[(B)/(C)] by mass of the component (B) to the component (C) ispreferably 1/99 to 90/10, more preferably 5/85 to 80/20, more preferably10/90 to 70/30, still more preferably 15/85 to 60/40, still much morepreferably 20/80 to 50/50, and particularly preferably 25/75 to 45/55.

In the lubricating oil composition of one embodiment of the presentinvention, from the viewpoint of obtaining a lubricating oil compositionhaving a good balance between cold startability and shear stability, thetotal content of the component (B) and the component (C) is preferably20.0 mass % or less, more preferably 16.0 mass % or less, still morepreferably 10.0 mass % or less, still much more preferably 8.0 mass % orless, and particularly preferably 6.0 mass % or less, and from theviewpoint of further improving the effect of suppressing cavitation, thetotal content of the component (B) and the component (C) is preferably0.2 mass % or more, more preferably 0.6 mass % or more, still morepreferably 1.0 mass % or more, still much more preferably 1.6 mass % ormore, and particularly preferably 2.0 mass % or more, based on the totalamount (100 mass %) of the lubricating oil composition.

Considering the handleability and the solubility with the component (A),the components (B) and (C) are often commercially available in a form ofa solution dissolved in a diluent oil.

However, in the present specification, the content of the components (B)and (C) is, in a solution diluted with a diluent oil, a content in termsof resin content constituting the component (B) or (C), excluding themass of the diluent oil.

The lubricating oil composition of one embodiment of the presentinvention may further contain lubricating oil additives other than thecomponents (B) to (C).

However, in the lubricating oil composition of one embodiment of thepresent invention, from the viewpoint of obtaining a lubricating oilcomposition in which the effect of suppressing cavitation and coldstartability are further improved, the total content of the components(A) to (C) is preferably 80 to 100 mass %, more preferably 85 to 100mass %, still more preferably 90 to 100 mass %, still much morepreferably 95 to 100 mass %, and particularly preferably 98 to 100 mass%, and further may be more than 98.5 mass % and 100 mass % or less, morethan 99.0 mass % and 100 mass % or less, more than 99.5 mass % and 100mass % or less, or more than 99.7 mass % and 100 mass % or less, basedon the total amount (100 mass %) of the lubricating oil composition.

Hereinafter, details of each component contained in the lubricating oilcomposition of one embodiment of the present invention will bedescribed.

<Component (A): Base Oil>

As the base oil which is the component (A) used in one embodiment of thepresent invention, one or more selected from mineral oils and syntheticoils can be mentioned.

Examples of the mineral oils include atmospheric residues obtained bysubjecting crude oils, such as paraffinic crude oil, intermediate basecrude oil and naphthenic crude oil, to atmospheric distillation;distillates obtained by subjecting these atmospheric residues to vacuumdistillation; and refined oils obtained by subjecting the distillates toone or more of refining treatments, such as solvent deasphalting,solvent extraction, hydrocracking, solvent dewaxing, catalytic dewaxing,and hydrorefining.

Examples of the synthetic oils include poly-α-olefins, such as anα-olefin homopolymer; isoparaffin; polyalkylene glycol; ester oils, suchas polyol ester, dibasic acid ester, and phosphoric acid ester; ether inoils, such as polyphenyl ether; alkylbenzene; alkylnaphthalene; andsynthetic oil (GTL) obtained by isomerizing wax (GTL WAX (Gas To LiquidsWAX)) produced from natural gas through Fischer-Tropsch process or thelike.

Among these, it is preferable to contain one or more selected frommineral oils classified in Group II and Group III of API (AmericanPetroleum Institute) base oil categories, and synthetic oils, as thecomponent (A) used in one embodiment of the present invention.

The kinematic viscosity of the component (A) used in one embodiment ofthe present invention at 40° C. is preferably 3.0 to 100 mm²/s, morepreferably 4.0 to 70 mm²/s, still more preferably 5.0 to 40 mm²/s, stillmuch more preferably 5.5 to 30 mm²/s, and particularly preferably 6.0 to20 mm²/s.

The viscosity index of the component (A) used in one embodiment of thepresent invention is appropriately set depending on the applications ofthe lubricating oil composition, and is preferably 70 or more, morepreferably 80 or more, still more preferably 90 or more, still much morepreferably 100 or more, and particularly preferably 105 or more.

When a mixed oil that is a combination of two or more base oils is usedas the component (A) in one embodiment of the present invention, thekinematic viscosity and the viscosity index of the mixed oil arepreferably in the above ranges.

In the lubricating oil composition of one embodiment of the presentinvention, the content of the component (A) is preferably 60 mass % ormore, more preferably 70 mass % or more, more preferably 75 mass % ormore, still more preferably 80 mass % or more, still much morepreferably 85 mass % or more, and particularly preferably 90 mass % ormore, and is preferably 99.8 mass % or less, more preferably 99.5 mass %or less, still more preferably 99.0 mass % or less, still much morepreferably 98.5 mass % or less, and particularly preferably 98.0 mass %or less, based on the total amount (100 mass %) of the lubricating oilcomposition.

<Component (B): Polyalkyl (Meth)Acrylate>

The lubricating oil composition of the present invention contains apolyalkyl (meth)acrylate having a weight average molecular weight (Mw)of 150,000 to 900,000 as the component (B). When Mw is less than150,000, the effect of suppressing cavitation is unlikely to beexhibited. When Mw is more than 900,000, it tends to be difficult toachieve both the effect of suppressing cavitation and shear stability.Since the component (B) and the component (C) described below are usedin combination, the lubricating oil composition can be adjusted to havegood shear stability.

The weight average molecular weight (Mw) of the polyalkyl (meth)acrylateused in one embodiment of the present invention as the component (B) is150,000 or more, and from the viewpoint of obtaining a lubricating oilcomposition in which the effect of suppressing cavitation and coldstartability are further improved, it is preferably 200,000 or more,more preferably 250,000 or more, more preferably 260,000 or more, morepreferably 270,000 or more, more preferably 300,000 or more, still morepreferably 320,000 or more, still more preferably 350,000 or more, stillmore preferably 370,000 or more, still more preferably 400,000 or more,still much more preferably 420,000 or more, still much more preferably450,000 or more, still much more preferably 470,000 or more, still muchmore preferably 500,000 or more, and particularly preferably 520,000 ormore. The weight average molecular weight (Mw) thereof is 900,000 orless, and from the viewpoint of obtaining a lubricating oil compositionhaving further improved shear stability, it is preferably 850,000 orless, more preferably 800,000 or less, more preferably 750,000 or less,still more preferably 700,000 or less, still much more preferably650,000 or less, and particularly preferably 600,000 or less.

In the lubricating oil composition of one embodiment of the presentinvention, from the viewpoint of obtaining a lubricating oil compositionin which the effect of suppressing cavitation and cold startability arefurther improved, the content of the component (B) is preferably 0.1mass % or more, more preferably 0.2 mass % or more, still morepreferably 0.3 mass % or more, still much more preferably 0.5 mass % ormore, and particularly preferably 0.7 mass % or more, and from theviewpoint of obtaining a lubricating oil composition having furtherimproved shear stability, the content of the component (B) is preferably10.0 mass % or less, more preferably 8.0 mass % or less, still morepreferably 5.0 mass % or less, still much more preferably 3.0 mass % orless, and particularly preferably 2.0 mass % or less, based on the totalamount (100 mass %) of the lubricating oil composition.

The component (B) used in one embodiment of the present invention may bea polymer having a structural unit derived from an alkyl acrylate or analkyl methacrylate (hereinafter, collectively referred to as “alkyl(meth)acrylate”), or may be a copolymer having a structural unit derivedfrom a monomer other than the alkyl (meth)acrylate.

The number of carbon atoms of the alkyl group in the alkyl(meth)acrylate may be 1 or more, 3 or more, 5 or more, or 10 or more,and may be 60 or less, 40 or less, 30 or less, or 20 or less.

In the component (B) used in one embodiment of the present invention,the content of the structural unit derived from the alkyl (meth)acrylatemay be 10 mol % or more, 30 mol % or more, 50 mol % or more, 60 mol % ormore, 70 mol % or more, 80 mol % or more, 90 mol % or more, 95 mol % ormore, or 99 mol % or more, based on the total amount (100 mol %) of thestructural unit of the component (B).

<<Comb-Shaped Polymer>>

The component (B) used in one embodiment of the present invention may bea comb-shaped polymer.

The comb-shaped polymer used in one embodiment of the present inventionas the component (B) is only required to be a polymer having a structureincluding a large number of three-way branch points from which a sidechain having a high-molecular weight comes out, in the main chain.

SSI (shear stability index) of the comb-shaped polymer used in oneembodiment of the present invention as the component (B) is preferably100 or less, more preferably 80 or less, still more preferably 70 orless, still much more preferably 60 or less, and particularly preferably50 or less.

The lower limit value of SSI of the comb-shaped polymer is notparticularly limited, and is usually 0.1 or more.

In the present specification, SSI (shear stability index) represents adecrease in viscosity caused by shear derived from the polymer componentby percentage, and is a value measured in accordance with JPI-5S-29-06,more specifically, a value calculated by the following expression (1).

SSI(%)=(Kv ₀ −Kv ₁)/(Kv ₀ −K _(voil))×100  Expression(1)

In the above expression (1), Kv₀ is a value of the kinematic viscosityof a sample oil at 100° C. in which the polymer component is diluted ina mineral oil, and Kv₁ is a value of the kinematic viscosity of thesample oil at 100° C. in which the polymer component is diluted in amineral oil, after being subjected to irradiation with ultrasonic wavefor 30 minutes by an output method in accordance with the procedures ofJPI-5S-29-06. Moreover, Kv_(oil) is a value of the kinematic viscosityof the mineral oil at 100° C. used when the polymer component isdiluted.

The value of SSI of the comb-shaped polymer varies with the structurethereof. Specifically, there are the following tendencies, and byconsidering these matters, the value of SSI of the comb-shaped polymercan be easily adjusted. The following matters are merely examples, andthe value of SSI of the comb-shaped polymer can also be adjusted byconsidering matters other than these matters.

-   -   A comb-shaped polymer whose side chain is constituted of the        macromonomer (x1) and in which the content of the structural        unit (X1) derived from the macromonomer (x1) is 0.5 mol % or        more, based on the total amount (100 mol %) of the structural        unit tends to have a low value of SSI.    -   As the molecular weight of the macromonomer (x1) constituting        the side chain of the comb-shaped polymer becomes higher, the        value of SSI tends to become lower.

The comb-shaped polymer used in one embodiment of the present inventionas the component (B) is preferably a polymer at least having astructural unit (X1) derived from a macromonomer (x1). This structuralunit (X1) corresponds to the aforementioned “side chain having ahigh-molecular weight”.

In the present specification, the above “macromonomer (x1)” means ahigh-molecular weight monomer having a polymerizable functional group,and is preferably a high-molecular weight monomer having a polymerizablefunctional group at a terminal thereof.

In the comb-shaped polymer used in one embodiment of the presentinvention as the component (B), the content of the structural unit (X1)is preferably 0.5 to 20 mol %, more preferably 0.7 to 10 mol %, andstill more preferably 0.9 to 5 mol %, based on the total amount (100 mol%) of the structural unit of the comb-shaped polymer.

In the present specification, the content of the structural unit in thecomb-shaped polymer means a value calculated by analyzing the ¹³C-NMRquantitative spectrum.

The number average molecular weight (Mn) of the macromonomer (x1) ispreferably 300 or more, more preferably 400 or more, still morepreferably 500 or more, and preferably 100,000 or less, more preferably50,000 or less, still more preferably 20,000 or less.

That is to say, the number average molecular weight (Mn) of themacromonomer (x1) is preferably 300 to 100,000, more preferably 400 to50,000, and still more preferably 500 to 20,000.

Examples of the polymerizable functional group included in themacromonomer (x1) include an acryloyl group (CH₂═CH—COO—), amethacryloyl group (CH₂═CCH₃—COO—), an ethenyl group (CH₂═CH—), a vinylether group (CH₂═CH—O—), an allyl group (CH₂═CH—CH₂—), an allyl ethergroup (CH₂═CH—CH₂—O—), a group represented by CH₂═CH—CONH—, and a grouprepresented by CH₂═CCH₃—CONH—.

In addition to the above polymerizable functional group, themacromonomer (x1) may have, for example, one or more repeating unitsrepresented by the following general formulae (i) to (iii).

In the above general formula (i), R^(b1) is a linear or branchedalkylene group having 1 to 10 carbon atoms.

In the general formula (ii), R^(b2) is a linear or branched alkylenegroup having 2 to 4 carbon atoms.

In the general formula (iii), R^(b3) is a hydrogen atom or a methylgroup. R^(b4) is a linear or branched alkyl group having 1 to 10 carbonatoms.

When the macromonomer (x1) has a plurality of repeating unitsrepresented by each of the above general formulae (i) to (iii), R^(b1),R^(b2), R^(b3) and R^(b4) may be each the same as one another or may bedifferent from one another.

In one embodiment of the present invention, the macromonomer (x1) ispreferably a polymer having a repeating unit represented by the generalformula (i), and more preferably a polymer having a repeating unit(X1-1) in which R^(b1) in the general formula (i) is at least oneselected from a 1,2-butylene group and a 1,4-butylene group.

The content of the repeating unit (X1-1) is preferably 1 to 100 mol %,more preferably 20 to 95 mol %, still more preferably 40 to 90 mol %,and still much more preferably 50 to 80 mol %, based on the total amount(100 mol %) of the structural unit of the macromonomer (x1).

When the macromonomer (x1) is a copolymer having two or more repeatingunits selected from the general formulae (i) to (iii), the form ofcopolymer may be a block copolymer or may be a random copolymer.

The comb-shaped polymer used in one embodiment of the present inventionas a component (B) may be a homopolymer consisting only of a structuralunit (X1) derived from one macromonomer (x1), or may be a copolymerhaving a structural unit (X1) derived from two or more macromonomers(x1).

The comb-shaped polymer used in one embodiment of the present inventionas a component (B) may be a copolymer having a structural unit (X2)derived from a monomer other than the macromonomer (x1) together with astructural unit (X1) derived from a macromonomer (x1).

As a specific structure of such a comb-shaped polymer, a copolymerhaving a side chain including the structural unit (X1) derived from themacromonomer (x1) relative to the main chain including the structuralunit (X2) derived from the monomer (x2) is preferable.

Examples of the monomer (x2) include alkyl (meth)acrylate, a nitrogenatom-containing vinyl monomer, a hydroxyl group-containing vinylmonomer, a phosphorus atom-containing monomer, an aliphatichydrocarbon-based vinyl monomer, a cycloaliphatic hydrocarbon-basedvinyl monomer, vinyl ester, vinyl ether, vinyl ketone, an epoxygroup-containing vinyl monomer, a halogen element-containing vinylmonomer, an ester of unsaturated polycarboxylic acid, (di)alkylfumarate, (di)alkyl maleate, and an aromatic hydrocarbon-based vinylmonomer.

The monomer (x2) is preferably a monomer other than the phosphorusatom-containing monomer and the aromatic hydrocarbon-based vinylmonomer, more preferably includes one or more selected from a monomerrepresented by the following general formula (al), alkyl(meth)acrylate,and a hydroxyl group-containing vinyl monomer, and still more preferablyincludes at least a hydroxyl group-containing vinyl monomer (x2-d).

In the general formula (al), R^(b11) is a hydrogen atom or a methylgroup.

R^(b12) is a single bond, a linear or branched alkylene group having 1to 10 carbon atoms, —O—, or —NH—.

R^(b13) is a linear or branched alkylene group having 2 to 4 carbonatoms. Moreover, n represents an integer of 1 or more (preferably aninteger of 1 to 20, and more preferably an integer of 1 to 5). When n isan integer of 2 or more, each R^(b13) may be the same as one another ormay be different from one another, and further, the (R^(b13)O) n moietymay be a random bond or a block bond.

R^(b14) is a linear or branched alkyl group having 1 to 60 (preferably10 to 50, and more preferably 20 to 40) carbon atoms.

<Olefin Copolymer (C)>

The lubricating oil composition of the present invention contains anolefin copolymer having a weight average molecular weight (Mw) of100,000 or less as the component (C). By using the olefin copolymerhaving Mw within the above range and the component (B) in combination, alubricating oil composition in which the effect of suppressingcavitation and shear stability are improved can be obtained. Since thecomponent (C) and the component (B) described above are used incombination, the lubricating oil composition can be adjusted to havegood cold startability.

The weight average molecular weight (Mw) of the olefin copolymer used inone embodiment of the present invention as the component (C) is 100,000or less, and from the viewpoint of obtaining a lubricating oilcomposition having further improved cold startability and shearstability while having a further improved effect of suppressingcavitation, it is preferably 80,000 or less, more preferably 70,000 orless, more preferably 60,000 or less, still more preferably 40,000 orless, still much more preferably 30,000 or less, and particularlypreferably 25,000 or less.

The weight average molecular weight (Mw) of the olefin copolymer as thecomponent (C) may be 500 or more, 1,000 or more, 3,000 or more, 5,000 ormore, 7,000 or more, 8,000 or more, more than 8,000, 8,500 or more,9,000 or more, 9,500 or more, 10,000 or more, 11,000 or more, 12,000 ormore, or 13,000 or more.

In the lubricating oil composition of one embodiment of the presentinvention, from the viewpoint of obtaining a lubricating oil compositionin which the effect of suppressing cavitation and shear stability arefurther improved, the content of the component (C) is preferably 0.1mass % or more, more preferably 0.3 mass % or more, still morepreferably 0.5 mass % or more, still much more preferably 0.8 mass % ormore, and particularly preferably 1.0 mass % or more, and from theviewpoint of obtaining a lubricating oil composition having furtherimproved cold startability, the content of the component (C) ispreferably 10.0 mass % or less, more preferably 8.0 mass % or less,still more preferably 5.0 mass % or less, still much more preferably 4.0mass % or less, and particularly preferably 3.0 mass % or less, andfurther may be 2.5 mass % or less, or 2.0 mass % or less, based on thetotal amount (100 mass %) of the lubricating oil composition.

The component (C) used in one embodiment of the present invention ispreferably a copolymer having a structural unit derived from a monomerhaving an alkenyl group, and examples thereof include a copolymer of anα-olefin having 2 to 20 (preferably 2 to 16, more preferably 2 to 14)carbon atoms, and more specific examples thereof include anethylene-α-olefin copolymer.

The number of carbon atoms of the α-olefin constituting theethylene-α-olefin copolymer is preferably 3 to 20, more preferably 3 to16, still more preferably 3 to 14, still much more preferably 3 to 6,and particularly preferably 3.

The component (C) used in one embodiment of the present invention may bea dispersion type olefin-based copolymer.

Examples of the dispersion type olefin-based copolymer include acopolymer obtained by graft-polymerizing the aforementionedethylene-α-olefin copolymer with maleic acid, N-vinyl pyrrolidone,N-vinyl imidazole, or glycidyl acrylate.

The component (C) used in one embodiment of the present invention may bea copolymer further having a structural unit derived from an aromaticmonomer together with a structural unit derived from a monomer having analkenyl group. Examples of such an olefin-based copolymer includestyrene-based copolymers such as a styrene-diene copolymer and astyrene-isoprene copolymer.

Among these, from the viewpoint of obtaining a lubricating oilcomposition in which the effect of suppressing cavitation and shearstability are improved, the component (C) used in one embodiment of thepresent invention preferably contains an ethylene propylene copolymer(C1).

In the lubricating oil composition of one embodiment of the presentinvention, the content ratio of the component (C1) is preferably 30 to100 mass %, more preferably 50 to 100 mass %, still more preferably 70to 100 mass %, still much more preferably 80 to 100 mass %, andparticularly preferably 90 to 100 mass %, based on the total amount (100mass %) of the component (C) contained in the lubricating oilcomposition.

<Lubricating Oil Additives>

The lubricating oil composition of one embodiment of the presentinvention may further contain lubricating oil additives other than thecomponent (B) and the component (C) when needed as long as the effectsof the present invention are not impaired.

Examples of such lubricating oil additives include an antioxidant suchas a phenol-based antioxidant and an amino-based antioxidant; ametal-based detergent such as metal sulfonate, metal salicylate, andmetal phenate; ashless dispersants such as alkenyl succinimide; afriction modifier such as a molybdenum-based friction modifier, a fattyacid ester, fatty acid, and an aliphatic alcohol; an anti-wear agentsuch as zinc dithiophosphate; an extreme-pressure agent such as aphosphorus-based extreme-pressure agent, a sulfur-based extreme-pressureagent, and a sulfur-phosphorus-based extreme-pressure agent; ananti-foaming agent such as a silicone-based anti-foaming agent; a metaldeactivator such as a benzotriazole-based compound; an anticorrosive;and an antistatic agent.

These lubricating oil additives may be each used singly, or may be eachused in combination of two or more.

The contents of these lubricating oil additives can be eachappropriately prepared as long as the effects of the present inventionare not impaired, but the contents of the additives are eachindependently usually 0.001 to 15 mass %, preferably 0.005 to 10 mass %,and more preferably 0.01 to 5 mass %, based on the total amount (100mass %) of the lubricating oil composition.

<Production Method for Lubricating Oil Composition>

The production method for the lubricating oil composition of oneembodiment of the present invention is not particularly limited, butfrom the viewpoint of productivity, preferable is a method having a stepof compounding the component (B) and the component (C), and ifnecessary, other lubricating oil additives with the component (A).

From the viewpoint of compatibility with the component (A), the resincomponent such as the component (B) and the component (C) is preferablyin a form of a solution dissolved in a diluent oil and the solution ispreferably compounded with the component (A).

[Properties of Lubricating Oil Composition]

The kinematic viscosity of the lubricating oil composition of oneembodiment of the present invention at 40° C. is preferably 5.0 to 130mm²/s, more preferably 6.5 to 100 mm²/s, still more preferably 8.0 to100 mm²/s, still much more preferably 10.0 to 60 mm²/s, and particularlypreferably 11.0 to 40 mm²/s.

The kinematic viscosity of the lubricating oil composition of oneembodiment of the present invention at 100° C. is preferably 2.0 to 30mm²/s, more preferably 2.3 to 20 mm²/s, still more preferably 2.6 to 15mm²/s, still much more preferably 3.0 to 10 mm²/s, and particularlypreferably 3.2 to 7.0 mm²/s.

The kinematic viscosity of the lubricating oil composition of oneembodiment of the present invention at 150° C. is preferably 1.0 to 20mm²/s, more preferably 1.2 to 10 mm²/s, still more preferably 1.4 to 7.0mm²/s, still much more preferably 1.6 to 5.0 mm²/s, and particularlypreferably 1.8 to 3.0 mm²/s.

The viscosity index of the lubricating oil composition of one embodimentof the present invention is preferably 100 or more, more preferably 120or more, still more preferably 140 or more, still much more preferably160 or more, and particularly preferably 180 or more.

From the viewpoint of obtaining a lubricating oil composition havinggood cold startability, the BF viscosity (Brookfield viscosity) of thelubricating oil composition of one embodiment of the present inventionat −40° C. is preferably 1,500 m·Pas or less, more preferably 1,400m·Pas or less, still more preferably 1,300 m·Pas or less, and still muchmore preferably 1,200 m·Pas or less, and is preferably 100 m·Pas ormore, more preferably 300 m·Pas or more, still more preferably 500 m·Pasor more, and still much more preferably 700 m·Pas or more.

In the present specification, the BF viscosity means a value measured inaccordance with ASTM D2983-09.

From the viewpoint of obtaining a lubricating oil composition havinggood shear stability, the kinematic viscosity reduction rate calculatedin accordance with the method described in Examples of the lubricatingoil composition of one embodiment of the present invention below ispreferably less than 11.0%, more preferably less than 10.5%, still morepreferably less than 10.0%, and still much more preferably less than9.5%.

From the viewpoint of obtaining a lubricating oil composition excellentin the effect of suppressing cavitation, the value of the cavitationfactor calculated in accordance with the method described in Examples ofthe lubricating oil composition of one embodiment of the presentinvention below is preferably 0.45 or less, more preferably 0.44 orless, and still more preferably 0.43 or less, and typically 0.40 ormore.

[Characteristics and Use Application of Lubricating Oil Composition]

The lubricating oil composition of one embodiment of the presentinvention has excellent characteristics such as cold startability, shearstability, and the effect of suppressing cavitation.

Thus, the lubricating oil composition of one embodiment of the presentinvention can be suitably used for lubrication of a shock absorber, andmore specifically, can be used for both a double cylinder type shockabsorber and a single cylinder type shock absorber, and can be suitablyused for both a shock absorber for two wheels and a shock absorber forfour wheels.

When these characteristics of the lubricating oil composition of oneembodiment of the present invention are taken into consideration, thepresent invention can also provide the following [1] and [2].

-   -   [1] A shock absorber filled with the aforementioned lubricating        oil composition of one embodiment of the present invention.    -   [2] Use of a lubricating oil composition, wherein the        aforementioned lubricating oil composition of one embodiment of        the present invention is applied to lubrication of a shock        absorber.

EXAMPLES

Next, the present invention will be described in much more detail withreference to Examples, but the present invention is in no way limited tothese Examples. Measuring methods for various properties are as follows.

(1) Kinematic Viscosity, Viscosity Index

The kinematic viscosity and viscosity index were measured and calculatedin accordance with JIS K2283:2000.

(2) Weight Average Molecular Weight (Mw)

Using a gel permeation chromatograph apparatus (manufactured by AgilentTechnologies, Inc., “1260 model HPLC”), the weight-average molecularweight was measured under the following conditions, and a value measuredin terms of standard polystyrene was used.

(Measurement Conditions)

Column: sequentially connected two of “Shodex LF404”.Column temperature: 35° C.Developing solvent: chloroformFlow rate: 0.3 mL/min

Example 1 and Comparative Examples 1 to 8

Each additive was compounded with the base oil in the types andcompounding amounts shown in Table 1, thereby preparing each lubricatingoil composition. The compounding amount of each additive described inTable 1 describes the compounding amount in terms of active ingredients(in terms of solid content) from which the mass of the diluent oil wasexcluded, even when each additive was compounded in a state beingdissolved in the diluent oil.

Details of the base oil and each additive used in the preparation ofeach lubricating oil composition are as follows.

<Base Oil>

“Paraffinic mineral oil”: paraffinic mineral oil, 40° C. kinematicviscosity=7.1 mm²/s, 100° C. kinematic viscosity=2.17 mm²/s, viscosityindex=109, 15° C. density=0.82 g/cm³.

<PMA>

“PMA (550,000)”: polyalkyl (meth)acrylate, weight average molecularweight (Mw)=550,000.

“PMA (29,000)”: polyalkyl (meth)acrylate, weight average molecularweight (Mw)=29,000.

“PMA (140,000)”: polyalkyl (meth)acrylate, weight average molecularweight (Mw)=140,000.

<OCP>

“OCP (17,000)”: ethylene propylene copolymer, weight average molecularweight (Mw)=17,000.

“OCP (780,000)”: olefin copolymer, weight average molecular weight(Mw)=780,000.

Regarding the lubricating oil composition prepared, the 40° C. kinematicviscosity, 100° C. kinematic viscosity, 150° C. kinematic viscosity, andviscosity index were measured or calculated in accordance with theaforementioned method, and the following measurement or evaluation wascarried out. The results of them are set forth in Table 1.

(1) Evaluation of Cold Startability

The BF viscosity at −40° C. was measured in accordance with ASTMD2983-09.

It is deemed that as the BF viscosity becomes lower, the lubricating oilcomposition has more excellent cold startability. When the BF viscosityat −40° C. was 1,500 m·Pas or less, the lubricating oil composition wasdetermined to have excellent cold startability.

(2) Evaluation of Shear Stability

The shear test was carried out in accordance with the ultrasonic Amethod (JPI-5S-29) by using the lubricating oil composition prepared asthe sample oil and irradiating 30 mL of the sample oil with ultrasonicwaves at 25° C. for 60 minutes. As the output voltage of ultrasonicwaves, the value of an output voltage at which the reduction rate of the40° C. kinematic viscosity after 30 mL of a standard oil whose 40° C.kinematic viscosity was measured in advance was irradiated withultrasonic waves at 25° C. for 60 minutes was 25% was employed.

Then, the 40° C. kinematic viscosity of the sample oil was measuredbefore and after the shear test, and the kinematic viscosity reductionrate was calculated by the following expression, thereby evaluating theshear stability.

Kinematic viscosity reduction rate (%)=([40° C. kinematic viscosity ofsample oil before shear test]−[40° C. kinematic viscosity of sample oilafter shear test])/[40° C. kinematic viscosity of sample oil beforeshear test]×100

It is deemed that as the value of the kinematic viscosity reduction ratebecomes smaller, the lubricating oil composition has more excellentshear stability. When the value of the kinematic viscosity reductionrate was less than 11.0%, the lubricating oil composition was determinedto have excellent shear stability.

(3) Evaluation of Effect of Suppressing Cavitation

FIG. 1 is a schematic diagram of a testing apparatus for evaluating theeffect of suppressing cavitation. A testing apparatus 1 shown in FIG. 1has an oil tank 11 that stores the sample oil of the subject to beevaluated, a pump 12 that circulates the sample oil, a heater 13 thatadjusts the temperature of the sample oil, a flowmeter 14 that controlsthe flow velocity of the sample oil, valves 15 a, 15 b, and 15 c thatcontrol the flow of the sample oil, an observation tank 16 provided withan observation window made of sapphire on a side surface and an orifice16 a with a diameter of 1 mm in an inlet on the upstream side, pressuregauges 17 a and 17 b on the upstream side and the downstream side of theobservation tank, and a thermocouple 18.

First, the oil tank 11 was filled with the lubricating oil compositionprepared as the sample oil, the valves 15 a, 15 b, and 15 c were fullyopened, the pump 12 and the heater 13 were operated, and the observationtank 16 was also filled with the sample oil while allowing the sampleoil to be circulated along with a flow path 1 and a flow path 2. Then,the state where the temperature of the sample oil reached 150° C. andstabilized was taken as the initial state.

From the initial state, while limiting the flow of the sample oil alongwith the flow path 2 by gradually closing the valve 15 c from thefully-opened state, the state where the pressure gauge 17 a on theupstream side exhibited “0.5 MPa” was taken as the starting point, andthe valve 15 a on the upstream side and the valve 15 b on the downstreamside were controlled to pressurize the sample oil in a stepwise manneruntil the pressure gauge 17 a on the upstream side exhibited “5.0 MPa”.In the pressurizing process, the degree of generation of cavitation wasvisually observed through a transparent window of the observation tank16, based on the cavitation score whose criteria were defined in advancedepending on the degree of generation of cavitation. At a point wherethe cavitation score becomes “5”, the pressure on the upstream side Pushown by the pressure gauge 17 a on the upstream side and the pressureon the downstream side Pd shown by the pressure gauge 17 b on thedownstream side were confirmed, thereby calculating the cavitationfactor.

The above cavitation score was evaluated by 11-grade in increments ofone, by taking no generation of cavitation as “0” and the state wherecavitation was most generated was “10”. The criteria at each grade werebased on the criteria defined in advance depending on the degree ofgeneration of cavitation.

The cavitation factor was calculated from the following expression, andthe value was set forth in Table 1.

Cavitation factor=(Pd+atmospheric pressure)/(Pu−Pd)

In the above expression, Pu is the pressure on the upstream side (unit:Pa) and Pd is the pressure on the downstream side (unit: Pa).

It is deemed that as the cavitation factor calculated from the aboveexpression becomes smaller, the lubricating oil composition has a highereffect of suppressing cavitation. When the value of the cavitationfactor was 0.45 or less, the lubricating oil composition was determinedto have an excellent effect of suppressing cavitation.

When cavitation with a cavitation score of “5” or more was alreadygenerated at the starting point at which the pressure on the upstreamside exhibits 0.5 MPa, cavitation was considered to be generated at alltimes, which is set forth in Table 1 as “F”.

TABLE 1 Comparative Comparative Comparative Comparative Example 1Example 1 Example 2 Example 3 Example 4 Composition Base oil Paraffinicmass % 97.23 100.00 96.00 98.46 91.74 mineral oil PMA PMA (550,000) mass% 0.77 — — 1.54 — PMA (29,000) mass % — — — — 8.26 PMA (140,000) mass %— — — — — OCP OCP (17,000) mass % 2.00 — 4.00 — — OCP (780,000) mass % —— — — — Total mass % 100.0 100.0 100.0 100.0 100.0 Properties 40° C.kinematic viscosity mm²/s 12.28 7.12 13.23 10.47 12.50 100° C. kinematicviscosity mm²/s 3.658 2.174 3.554 3.551 3.836 150° C. kinematicviscosity mm²/s 1.988 1.224 1.867 2.026 2.103 Viscosity index — 206 109159 264 228 Evaluation (1) Low BF viscosity mPa · s 1180 (*1) (*1) 8501420 temperature (−40° C.) fluidity (2) Shear Kinematic % 9.4 0.0 0.319.6 0.1 stability viscosity reduction rate (3) Effect of Cavitation —0.43 F 0.30 0.33 0.48 suppressing factor cavitation ComparativeComparative Comparative Comparative Example 5 Example 6 Example 7Example 8 Composition Base oil Paraffinic mass % 95.10 97.60 96.25 95.00mineral oil PMA PMA (550,000) mass % 0.77 — — — PMA (29,000) mass % 4.13— — — PMA (140,000) mass % — 2.40 — — OCP OCP (17,000) mass % — — — —OCP (780,000) mass % — — 3.75 5.00 Total mass % 100.0 100.0 100.0 100.0Properties 40° C. kinematic viscosity mm²/s 11.76 10.74 13.25 14.19 100°C. kinematic viscosity mm²/s 3.796 3.437 3.771 3.817 150° C. kinematicviscosity mm²/s 2.121 1.922 2.014 1.998 Viscosity index — 250 227 192173 Evaluation (1) Low BF viscosity mPa · s 1020 820 1610 (*1)temperature (−40° C.) fluidity (2) Shear Kinematic % 11.7 11.0 22.1 22.0stability viscosity reduction rate (3) Effect of Cavitation — 0.49 F0.34 F suppressing factor cavitation (*1) Solidified and unmeasurable

As shown in Table 1, the lubricating oil composition prepared in Example1 has excellent cold startability and shear stability, resulting in ahigh effect of suppressing cavitation. On the other hand, thelubricating oil compositions prepared in Comparative Examples 1 to 8resulted in having poor cold startability, poor shear stability, or apoor effect of suppressing cavitation.

REFERENCE SIGNS LIST

-   -   1 Testing apparatus    -   11 Oil tank    -   12 Pump    -   13 Heater    -   14 Flowmeter    -   15 a, 15 b, 15 c Valve    -   16 Observation tank    -   16 a Orifice    -   17 a, 17 b Pressure gauge    -   18 Thermocouple

1. A lubricating oil composition suitable for lubrication of a shockabsorber, the composition comprising: a base oil (A); a polyalkyl(meth)acrylate (B) having a weight average molecular weight in a rangeof from 150,000 to 900,000; and an olefin copolymer (C) having a weightaverage molecular weight of 100,000 or less.
 2. The composition of claim1, wherein the polyalkyl (meth)acrylate (B) and the olefin copolymer(C), together, are present in is 20.0 mass % or less, based on totallubricating oil composition mass.
 3. The lubricating oil composition ofclaim 1, wherein a (B)/(C) mass content ratio of the polyalkyl(meth)acrylate (B) to the olefin copolymer (C) is in a range of from1/99 to 90/10.
 4. The lubricating oil composition of claim 1, whereinthe polyalkyl (meth)acrylate (B) is present in a range of from 0.1 to10.0 mass %, based on total lubricating oil composition mass.
 5. Thelubricating oil composition of claim 1, wherein the olefin copolymer (C)is present in a range of from 0.1 to 10.0 mass %, based on totallubricating oil composition mass.
 6. The lubricating oil composition ofclaim 1, wherein a weight average molecular weight of the olefincopolymer (C) is in a range of from 8,000 to 100,000.
 7. The lubricatingoil composition of claim 1, wherein the olefin copolymer (C) comprisesan ethylene propylene copolymer (C1).
 8. The composition of claim 1,wherein the components (A), polyalkyl (meth)acrylate (B), and the olefincopolymer (C), together, are present in a range of from 80 to 100 mass %based on the total amount of the lubricating oil composition.
 9. Thelubricating oil composition of claim 1, having a BF viscosity at-40° C.is 1,500 m·Pas or less.
 10. A method of lubricating a shock absorber,the method comprising: applying the lubricating oil composition of claim1, to the shock absorber.
 11. The composition of claim 2, wherein a(B)/(C) mass content ratio of the polyalkyl (meth)acrylate (B) to theolefin copolymer (C) is in a range of from 1/99 to 90/10.
 12. Thecomposition of claim 2, wherein the polyalkyl (meth)acrylate (B) ispresent in a range of from 0.1 to 10.0 mass %, based on totallubricating oil composition mass.
 13. The composition of claim 3,wherein the polyalkyl (meth)acrylate (B) is present in a range of from0.1 to 10.0 mass %, based on total lubricating oil composition mass. 14.The composition of claim 11, wherein the polyalkyl (meth)acrylate (B) ispresent in a range of from 0.1 to 10.0 mass %, based on totallubricating oil composition mass.
 15. The composition of claim 14,wherein the olefin copolymer (C) is present in a range of from 0.1 to10.0 mass %, based on total lubricating oil composition mass.